A new deadline for an agreement is looming tomorrow, and both sides are asking how much longer busy ministers can clear their schedules to hunker down in Vienna as they seek to end a 13-year standoff with the Islamic republic.

"Everything is on the table. It's now time to decide," French Foreign Minister Laurent Fabius yesterday warned on the 15th day of the tortuous talks.

Iran and the so-called P5+1 group, including Britain, China, France, Germany, Russia and the United States, are seeking to curtail Tehran's ability to develop nuclear weapons in exchange for relief from painful sanctions.

Three previous deadlines in this round of talks - June 30, July 7 and July 9 - have already been missed and officials at the negotiations said sanctions and an arms embargo remain key sticking points.

"Ninety-eight percent of the text is finished," said a source close to the discussions after a flurry of bilateral and multilateral talks throughout Saturday.

Two or three important questions remained, however, including how long any agreement made in Vienna should last, and over the lifting of international sanctions on Iran such as a UN arms embargo, the source added.

"Now there needs to be a political decision. And if that is taken things could quickly" progress, the person said.

US Secretary of State John Kerry tweeted that there were still "difficult issues to resolve" after emerging from almost 90 minutes of talks with his Iranian counterpart Mohammad Javad Zarif and EU foreign policy chief Federica Mogherini.

The three met again later in the evening, in between two meetings of the P5+1 group, which ended just before midnight. Kerry had spoken earlier by telephone with Russian Foreign Minister Sergei Lavrov, but it remained unclear when Lavrov would return to the Austrian capital.

Under the parameters of a framework deal reached in Lausanne in April, Iran is to slash the number of its centrifuges from more than 19,000 to just over 6,000 and cut its stocks of enriched uranium, which can be used to make a bomb, from more than seven tonnes to about 350 kilos.